1. Field of the Invention
The present invention relates generally to fluid pressure sensors, and particularly to capacitive fluid pressure sensors.
2. Technical Background
Capacitive sensors are used widely in a variety of medical and industrial applications to measure linear parameters such as force and pressure. For example, capacitive pressure sensors are used in blood pressure products. They can also be used to measure air or other fluid pressures.
In the conventional approach, the capacitive pressure sensor includes a pressure sensor assembly coupled to an electronic circuit. The pressure sensor assembly includes a port that connects the sensor to the environment. The sensor itself is constructed using a variable capacitor. In this design, one plate of the variable capacitor is formed by attaching a metal plate to a diaphragm. A fixed plate is held at a distance using spacer elements, that are fixed or adjustable. There are several problems with the conventional approach.
Fabrication is difficult and expensive. The variable capacitor is not an off-the-shelf component. The use of a metal plate attached to a diaphragm introduces measurement errors if the planar surface of the attached plate is not parallel to the planar surface of the fixed plate. Further, the attached metal plate has a relatively large mass. This substantially increases the sensor""s susceptibility to errors caused by vibration, acceleration, and the sensor""s orientation to the earth""s gravitational field.
The electronic circuit used in the conventional approach employs a three-inverter oscillator circuit that converts the capacitance of the capacitive transducer to a square wave. The frequency of the square wave is easily measured by a microprocessor, or by some other means. FIG. 1 is an electrical schematic of a conventional three-inverter oscillator circuit. The circuit includes three inverter gates G1, G2, and G3. Typically, each gate includes protection diodes. The biggest problem with the circuit depicted in FIG. 1 is the conduction of the input protection diodes of the threshold detector stage G1. In order to mitigate the effects of the diode conduction, the conventional design employs resistor R3. Depending on its value, R3 either reduces or eliminates the diode conduction. However, as R3 reduces diode errors, it amplifies errors introduced by other components in the oscillator. The direct effect is increased sensitivity to changes in capacitance of the internal circuit at the input of gate G1, thus affecting frequency stability. The group delay of the low-pass filter created by R3 and the input capacitance of G1 causes the sensitivity to most other errors in the sensor to be increased. The ideal situation is where no delays are added to the signal path. The introduction of R3 also changes the effective threshold voltage (Vth) of gate G1. The conventional design has other problems as well. The circuit board that is used to support the electronics and the means used to support the plates of the variable capacitor C1 include dielectric material that contributes to inter-plate capacitance between the nodes of the circuit, especially between plates of the capacitor C1. Because the dielectric constant of the circuit board supports varies with temperature, the conventional sensor is sensitive to changes in temperature.
What is needed is a compact, inexpensive capacitive sensor that is easily fabricated using commonly available components. A sensor is needed that is not susceptible to errors caused by vibration, acceleration, and sensor orientation to the earth""s gravitational field. A sensor is needed that includes an improved oscillator circuit that reduces the effects of diode conduction, including drift over temperature, without the errors introduced by the conventional design.
The present invention relates to a capacitive sensor that is compact, inexpensive and easily fabricated using commonly available components. The capacitive sensor of the present invention is less susceptible to errors caused by vibration, acceleration, or its orientation to the earth""s gravitational field. The capacitive sensor of the present invention includes an improved oscillator circuit that reduces the effects of diode conduction without the errors of the conventional design. Thus, the output of the sensor of the present invention does not substantially drift over temperature.
One aspect of the present invention is a capacitive sensor for measuring a stimulus parameter. The sensor includes a circuit board having at least one metallic layer. A metallic diaphragm is coupled to the circuit board and juxtaposed to the metallic layer to thereby form a transducer capacitor characterized by a capacitance. The metallic diaphragm is adapted to move relative to the at least one metallic layer in response to a change in the stimulus parameter such that the capacitance changes in accordance with the change in the stimulus parameter. An oscillator circuit including a low-pass filter is coupled to the transducer capacitor. The oscillator circuit is configured to generate a filtered signal characterized by a frequency. The frequency changes in accordance with capacitance changes.
In another aspect, the present invention includes a capacitive sensor for measuring a stimulus parameter. The sensor includes a capacitor transducer including a fixed plate member and a variable plate member. The capacitor transducer is characterized by a variable capacitance. The variable capacitance varies in accordance with a change in the stimulus parameter. An oscillator circuit is coupled to the capacitor transducer. The oscillator circuit includes a low-pass filter coupled to an input of the capacitive transducer. The oscillator circuit generates a non-sinusoidal signal having a frequency. The frequency is proportional to the stimulus parameter.
In another aspect, the present invention includes a capacitive sensor system for measuring a stimulus parameter. The system includes a circuit board having at least one metallic layer disposed therein. A metallic diaphragm is coupled to the circuit board to thereby form a variable capacitor. The variable capacitor is characterized by a variable capacitance. The metallic diaphragm is adapted to move relative to the at least one metallic layer in response to a change in a stimulus parameter, such that the capacitance is varied in accordance with stimulus parameter changes. An oscillator circuit is disposed on the circuit board and coupled to the variable capacitor. The oscillator circuit includes a low-pass filter configured to generate a filtered signal characterized by a frequency that changes in accordance with capacitance changes. A processor is coupled to the oscillator circuit. The processor is configured to derive a value of the stimulus parameter from the frequency.
In another aspect, the present invention includes a method for calibrating a capacitive sensor used to measure a stimulus parameter. The method includes providing a sensor including a capacitor transducer and an oscillator circuit, the capacitor transducer being characterized by a variable capacitance that varies in accordance with a change in the stimulus parameter. A correction factor is determined by comparing an initial condition to an ambient condition. The frequency corresponding to the stimulus parameter is determined during ambient conditions. The stimulus parameter is corrected by multiplying the correction factor by the frequency, whereby a corrected frequency value is obtained.
In another aspect, the present invention includes a capacitive sensor for measuring a stimulus parameter. The sensor has a circuit board including at least one metallic layer. A metallic diaphragm is coupled to the circuit board and juxtaposed to the metallic layer to thereby form a transducer capacitor characterized by a capacitance. The metallic diaphragm becomes substantially curved relative to the at least one metallic layer in response to a change in the stimulus parameter such that the capacitance changes in accordance with stimulus parameter changes. An oscillator circuit is coupled to the transducer capacitor. The oscillator circuit is configured to generate a signal characterized by a frequency that changes in accordance with capacitance changes.
In another aspect, the present invention includes a capacitive sensor for measuring a stimulus parameter. The sensor has a circuit board including at least one metallic layer. A metallic diaphragm is coupled to the circuit board and juxtaposed to the metallic layer to thereby form a transducer capacitor characterized by a capacitance. The metallic diaphragm is adapted to move relative to the at least one metallic layer in response to a change in the stimulus parameter such that the capacitance changes in accordance with stimulus parameter changes. A conductive ring is disposed between the metallic diaphragm and the circuit board. A pressure port assembly is coupled to the conductive ring, whereby a cavity is formed between a pressure port and the metallic diaphragm. An oscillator circuit is coupled to the transducer capacitor. The oscillator circuit is configured to generate a signal characterized by a frequency that changes in accordance with capacitance changes.
In another aspect, the present invention includes a capacitive sensor for measuring a stimulus parameter. The sensor has a circuit board including at least one metallic layer. A metallic diaphragm is coupled to the circuit board and juxtaposed to the metallic layer to thereby form a transducer capacitor characterized by a capacitance. The metallic diaphragm is adapted to move relative to the at least one metallic layer in response to a change in the stimulus parameter such that the capacitance changes in accordance with stimulus parameter changes. At least one guard ring is disposed within a thickness of the circuit board. The guard ring is adapted to reduce stray capacitance between the metallic diaphragm and the metallic layer. An oscillator circuit is coupled to the transducer capacitor. The oscillator circuit is configured to generate a signal characterized by a frequency that changes in accordance with capacitance changes.
In another aspect, the present invention includes a capacitive sensor for measuring a stimulus parameter. The sensor has a circuit board including at least one metallic layer. A metallic diaphragm is coupled to the circuit board and juxtaposed to the metallic layer to thereby form a transducer capacitor characterized by a capacitance. The metallic diaphragm does not include an attached metallic plate. The metallic diaphragm is adapted to move relative to the at least one metallic layer in response to a change in the stimulus parameter such that the capacitance changes in accordance with stimulus parameter changes. An oscillator circuit is coupled to the transducer capacitor. The oscillator circuit is configured to generate a signal characterized by a frequency that changes in accordance with capacitance changes.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.